Computer systems (and related devices) typically create three-dimensional images using a sequence of stages known as a graphics pipeline. During early pipeline stages, images are modeled using a mosaic-like approach where each image is composed of a collection of individual points, lines and polygons. These points, lines and polygons are know as primitives and a single image may require thousands, or even millions, of primitives. Each primitive is defined in terms of its shape and location as well as other attributes, such as color and texture.
The graphics pipeline maps each primitive into a memory storage device know as a frame buffer. Each storage location within the frame buffer defines one pixel within the image being produced. The graphics pipeline performs the mapping process by determining which pixels (i.e., which frame buffer storage locations) are included within each primitive. Each pixel is then initializes to reflect the attributes of the primitive, or primitives in which it is included. In many cases, the graphics pipeline will further modify the pixel values in the frame buffer to apply texture, lighting and other effects to the graphics primitives.
The task of rendering primitives to pixels can be very time consuming. This is especially true for complex images that include many primitives or require complex lighting, shading or other effects. The time consumed transforming primitives becomes problematic for applications, such as flight simulators and virtual reality environments, where rapid image generation is required.
One method for improving the speed of the rendering process is to use multiple graphics pipelines. For this method, the task of rendering primitives is subdivided so that each pipeline performs some portion of the total task. One way of subdividing the rendering task is to assign a fixed portion of each image to a different pipeline. The pipelines operate in parallel and each pipeline processes the primitives that are included in its image portion.
The parallel output of the various pipelines is combined for display. To be effective, this combination must be accomplished seamlessly, as though the image were being created by a single graphics pipeline. In general, this seamless integration requires that the output of each pipeline be synchronized. Otherwise, the pipelines produce output at somewhat random intervals. This can make the resulting images appear to be inaccurate or even incoherent.
For some systems, the synchronization problem may be overcome through the use of a global ready signal. SILICON GRAPHICS INC. provides a signal of this type, known as SWAP_READY, for use with certain types of SGI computers. The SWAP_READY signal is physically connected to each graphics pipelines that is to be synchronized. Systems that are ready to output their image portion assert local ready signals. SWAP_READY functions as a logical AND of these local ready signal. As a result, SWAP_READY becomes asserted when all systems are ready to output their image portions. The graphics pipelines sample the state of the SWAP_READY signal on a periodic, synchronized basis. The pipelines output their image portions when the discover that SWAP_READY is high.
A second way to combine multiple graphics pipelines is to subdivide the rendering task so that the pipelines operate in round-robin or sequential fasion. For this method, each pipeline contributes a succeeding image to a sequence of images. The sequential method has certain advantages when compared to the method where each image is partitioned between pipelines. One of these advantages is the ability to use less powerful graphics pipelines to produce a sequence of images at an accelerated rate. This is especially beneficial for flight simulators and other real time environments.
To ensure image quality, sequential pipeline operation requires synchronization of the pipelines that are used to generate an image. Otherwise, there is a chance that the pipelines will output images out of order. For animation and simulation environments, this can cause the display to move unpredictably forward and backward in time. Unfortunately, a synchronization method, such as a global ready signal, is not available for systems using the sequential pipeline operation. As a result, complex software synchronization is required, increasing the difficulty of implementing systems of this type.
For some applications it is desirable to use a combination of both of the previously described techniques for combining multiple graphics pipelines. This allows images to be partitioned into regions with each region using one or more pipelines connected in sequence. The sequential operation of each group of one or more pipelines proceeds in parallel with the operation with the remaining pipeline groups. This provides a flexible method for arranging graphics pipelines to match the image generation requirements of specific applications. Unfortunately, the lack of a synchronization method for sequential pipeline operation makes combination of these systems with other graphics pipelines difficult.
Based on the preceding it may be appreciated that a need exists for an effective method for synchronizing multiple graphics pipelines that are configured to operate in a sequential fashion. A preferable solution to this problem would allow sequential graphics pipeline groups to operate in parallel with other pipeline groups.